1. Field of the Invention
This invention relates to a process and a device for testing of a workpiece by means of eddy currents which are induced by a field coil in the workpiece and from which a measurement signal is obtained by means of a measurement sensor.
2. Description of Related Art
Testing of an electrically conductive workpiece by means of eddy currents relies upon the fact that material faults in the workpiece or test piece (for example, cracks, voids, surface damage, poor welds, etc.) hinder the propagation of eddy currents which are induced by means of the field coil; this acts on the electromagnetic field which has been formed in turn by the eddy currents. This electromagnetic field which has been produced by the eddy currents is measured by means of a sensor which can be a field coil itself or at least one separate measurement coil. If there is only a single separate measurement coil, this arrangement is called an xe2x80x9cabsolute coilxe2x80x9d. Two or more measurement coils can be connected in a subtractive manner; this is called a xe2x80x9cdifference coilxe2x80x9d and enables for example a temperature drift to be excluded. A material fault which influences the eddy currents can thus be considered an impedance change of the measurement coil which is coupled to the test piece, the fault signals being conventionally displayed as a vector quantity (amplitude and phase angle) in the impedance plane of the measurement coil. The phase shift of the eddy currents with respect to the exciter voltage depends among other things on the thickness of the material. The exciter frequency determines the penetration depth and the phase shift of the eddy currents in a certain material depth.
Examples of phase-selective evaluation of eddy current measurements for material testing can be found for example in published European Patent Application No. 0 282 930, and in U.S. Pat. Nos. 4,646,013, 4,355,281, 4,853,634, and 5,371,462 discloses eliminating measurement signals which originate not from faults, but from the effects of the geometry of the test piece, for example edges, by generating a reference background signal by scanning a fault-free workpiece identical to the test specimen, which signal is withdrawn from the measurement signal obtained for the test specimen after the actual measurement of the latter.
Basically, the evaluation of the eddy current signals can be prevented or hindered by noise signals (this includes generally interference signals of all types which can also be periodic) which can be produced by mechanical or electromagnetic interference (for example, by a welding machine in the line). In addition, the surface of the test specimen can lead to increased background noise when it is for example a galvanized surface. In practice, the attempt has been made to counter these difficulties by modification of the exciter frequency, multi-frequency processes, or modification of the filters. However, it has been shown that existing measurement and evaluation processes do not always lead to sufficiently reliable and reproducible results. In particular, it would be desirable to detect fault signals in the vicinity of or below the noise level as well.
An object of this invention is to devise a process and a device for testing a workpiece by means of eddy currents in which faults can be detected with greater reliability and accuracy than in the known methods.
This object is achieved by a process and a device as disclosed hereinafter. It is advantageous that the reliability of fault detection can be greatly increased principally for very noisy measurement signals by evaluating the correlation of the measurement signal to a pattern signal representative or typical of a workpiece fault instead of the actual measurement signal.